3.0.1b5

• Remove six package from code and dependencies (remove support python2)
• Use anonymous credentials by default instead of iam metadata (use ydb.driver.credentials_from_env_variables for creds by env var)
• Close grpc streams while closing readers/writers
• Add control plane operations for topic api: create, drop
• Add six package to requirements

Full Changelog: 3.0.1b4...3.0.1b5